Civilian casualties have running at near record highs as fighting spreads to more areas of Afghanistan, according to the United Nations.

Afghan President Ashraf Ghani generally has been less vocal than his predecessor, Hamid Karzai, in publicly criticising the U.S. military when troops are involved in incidents where civilians are killed.

On Saturday, three American soldiers were killed and one wounded when an Afghan soldier opened fire on them in Nangarhar, where elite U.S. troops have been helping Afghan forces battle Islamic State militants.

Also over the weekend, an American air strike in southern Afghanistan killed at least three Afghan policemen and wounded several others during a joint operation by Afghan and U.S. special forces. U.S. and Afghan troops have been battling militants in Nangarhar province for months.

The Islamic State militant group (ISIS), or Daesh as it is generally known in Afghanistan, has established a stronghold in the region, which borders Pakistan. U.S. military officials estimate there are about 600 to 800 Islamic State fighters in Afghanistan, mostly in Nangarhar, but also in the neighboring province of Kunar.

The increase in involvement by U.S troops and warplanes comes as U.S. President Donald Trump's administration weighs whether to deploy more troops in the war-torn country.

Reuters reported in late April that the U.S. administration was carrying out a review of Afghanistan and there were conversations over whether to send between 3,000 and 5,000 U.S. and coalition troops to Afghanistan.